The present application relates to power semiconductor devices, methods, and circuits, and more particularly to a power MOSFET with embedded recessed field plate and methods of fabrication thereof.
Power MOSFETs are widely used as switching devices in electronic applications. In order to minimize conduction power loss it is desirable that power MOSFETs have a low specific on resistance (Rsp), which is defined as the product of the on-resistance (Ron) of the MOSFET multiplied by the active die area A of the MOSFET (Ron*A). In general, the on-resistance of a power MOSFET is dominated by the channel resistance and the drift region resistances which include the channel resistance, spreading resistance and the epitaxial layer resistance.
The most common way to reduce the Rsp is to shrink the device's unit cell pitch and increase the packing density or number of cells per unit area. However, as the cell density increases, the associated intrinsic capacitances of device, such as the gate-to-source capacitance (Cgs), the gate-to-drain capacitance (Cgd), the total input capacitance (Ciss) and the total output capacitance (Coss), also increase. Therefore, for higher cell density devices, the switching power loss of device will increase. In many switching applications such as the synchronous buck dc-dc converters used in mobile products, MOSFETs are required to operate at high switching frequencies approaching the range of 1 MHz or higher. Therefore, it is required to minimize the switching or dynamic power loss governed by these capacitances.
It is also, of course, desirable to maximize the breakdown voltage of a power transistor. Increased breakdown voltage and lower specific on-resistance are both desirable. There are many trade-offs in optimizing both of these for particular applications. However, for a given specific on-resistance, higher breakdown voltage is generally desirable, and for a given breakdown voltage, lower specific on-resistance is generally desirable.
Another generally desirable property is ruggedness: if a device sees an overvoltage beyond its limits, it is desirable that the device should become functional again after that.
Another desirable property, in most cases, is switching speed. This depends on various factors, but one of them is the amount for Reverse Recovery charge QRR (per unit area at the surface) of the body-drain diode which must be removed to turn the device off.